1. Field of the invention
The present invention relates to the field of devices for heating a subterranean formation.
2. Background Information
The utility and desirability of applying heat to subterranean formations are well known. There are numerous applications, including oil production and remediation of contaminated soils. In many instances it is desirable to heat thick underground sections as uniformly, efficiently, and economically as possible. There are vast deposits of hydrocarbons all over the world, including oil shales, tar sands, and oil-bearing diatomites that will yield combustible gases and oil when heated. There are thousands of sites that have been contaminated by pollutants that can be driven out of the soil or decomposed in place by the application of heat. There may be numerous other uses for the application of subterranean heat as well, including, but not limited to: the accelerated digestion of landfills, thawing of permafrost, gasification of coal, production of methane hydrates, and others.
A means for heating subterranean formations was invented by Ljungstrom in 1940, U.S. Pat. No. 2,732,195. Ljungstrom""s invention pertained to formations of oil shale and used electrical resistance heaters installed in a pattern of vertical bore holes to heat the formation through thermal conduction. When heat was applied to the oil shale formation the waxy hydrocarbon xe2x80x9ckerogenxe2x80x9d began to break down to yield fuel gases and shale oil. Heat induced pressures in the formation drove these products into collection wells, where they were recovered. The so-called xe2x80x9cLjungstrom Methodxe2x80x9d was successfully deployed in Sweden during World War II to help alleviate a critical shortage of liquid fuels in that country. Details regarding the Ljungstrom Method have been widely published in journal articles like xe2x80x9cUnderground Shale Oil Pyrolysis According to the Ljungstrom Method, xe2x80x9d by G. Salomonsson, Swedish Shale Oil Corp., IVA, vol. 24, (1953), No. 3, pp. 118-123. xe2x80x9cProduction of Shale Oil in Sweden,xe2x80x9d by H. E. Linden, Producer""s Monthly, July, 1948, pp. 29-34. And others.
Since Ljungstrom, there have been numerous inventions for other types of apparatus and methods to heat subterranean formations. These include: gas fired burnersxe2x80x94U.S. Pat. No. 2,902,270; elongated porous combustion tubesxe2x80x94U.S. Pat. No. 3,113,623; catalytic heatersxe2x80x94U.S. Pat. No. 3,804,163; electro-heating with electrodesxe2x80x94U.S. Pat. No. 4,412,585; heating with radio frequency electromagnetic radiation (microwaves)xe2x80x94U.S. Pat. No. 4,320,801; flameless combustorsxe2x80x94U.S. Pat. No. 5,255,742; circulation of hot combustion gasesxe2x80x94U.S. Pat. No. 6,056,057; and nuclear reactor cooling fluidsxe2x80x94U.S. Pat. No. 3,237,689.
Many of the above inventions apply to oil shales, oil sands, coal seams or other hydrocarbon formations. Heating has numerous effects which aid in the recovery of a variety of fossil resources: Heat reduces the viscosity of heavy oils, produces fuel gases through distillation, pressurizes underground formations, and forms fractures due to gas and steam pressure and thermal expansion. According to U.S. Pat. No. 5,297,626: xe2x80x9cProduction of oil in a thermal conduction process is by pressure drive, vaporization and thermal expansion of oil and water trapped within the pores of the formation rock. Oil migrates through small fractures created by the expansion and vaporization of the oil and water.xe2x80x9d
One of the great advantages of heating the ground to facilitate resource extraction is the fact that it is a so-called xe2x80x9cin-situxe2x80x9d process. In-situ means that the resource ore body is left in place in the ground while the oil, gas, or other desirable products are removed. This has great advantages over mining or other xe2x80x9cex-situxe2x80x9d techniques, which require physical removal of the ore bodies, and extensive processing to separate the desired products from their mineral matrix. Insitu processes minimize capital expenditures and are generally less environmentally disruptive. In-situ processes also tend to be scalable, allowing projects to start small and grow incrementally.
The opposite is frequently true of mining ventures, which can require a minimum size for economical operation. This has especially been the case with hydrocarbon resources like oil shale, where immense mining operations would be required in order to achieve economies of scale that would allow shale oil to be produced at competitive costs. It has been estimated that production of one million barrels of shale oil per day would require mining activity equivalent to all other U.S. mines put together. Building such enormous facilities means taking huge financial risks before the first dollar of revenue can be realized. This fact, together with the environmental complications of such projects, has prevented their being brought to fruition in the past.
In-situ heating of underground formations can also be used to decontaminate polluted soils. U.S. Pat. Nos. 5,318,116 and 5,244,310, for example, disclose methods for decontaminating soils by injecting heat below the surface in order to break down, vaporize, and mobilize pollutants.
All of the previously proposed in-situ methods for applying subterranean heat have suffered from serious disadvantages. For example, those that require inputs of electrical energy, like electrical resistance heaters and microwave electrodes, face serious thermodynamic and economic inefficiencies. According to U.S. Pat. No. 5,297,626: xe2x80x9cThe high cost of electrical energy is also an impediment to commercial projects using these prior art methods. Conversion of hydrocarbons to electrical energy is typically accomplished at only about 35 percent efficiency and requires a considerable capital investment.xe2x80x9d
Essentially, electrically powered heaters trade electricity for heat. The electricity, however, must first be produced, usually through combustion of some fuel. Since typical central power plant efficiencies are only 30-35%, this means that every BTU of heat put in the ground by these methods may require 3 BTU of fuel consumed in power plants. The Ljungstrom Method, for example, requires 24 kilowatt hours (kWh) of power for every gallon of oil produced. Producing 24 kWh of electricity in a central power plant might require 254,000 BTU worth of fuel. Since a gallon of oil typically contains 140,000 BTU, it can be seen that the Ljungstrom Method operated at a loss.
The Ljungstrom Method was used briefly in Sweden only because that country had an abundance of cheap hydropower and an acute shortage of liquid transportation fuels. Other electrically powered systems for ground heating are reportedly less demanding of electricity than the Ljungstrom Method, but are still uneconomic. For example, heating of an oil shale formation by microwaves is reported to yield one gallon of oil for every 9.3 kWh of electricity expended. (R. Mallon, Economics of Shale Oil Production by Radio Frequency Heating, Report No. UCRL-52942, Lawrence Livermore Laboratory, May 1980, p. 134.) At prevailing rates of $0.05/kwh, this would translate into a cost of $19.50 per barrel for electricity alone. (These calculations do not take into account the production of noncondensable gases, or other co-products that may or may not have economic value.)
Other subterranean heaters in the prior art attempt to address the inefficiencies inherent in electrically driven systems by burning fuels to produce heat directly. Generally heat from combustion is cheaper than heat from electricity. These systems avoid the intermediate step of producing electricity and therefore are thermodynamically superior, but still suffer from economic shortcomings that have thus far prevented their widespread adoption. Inventions of this type include gas-fired heaters, catalytic heaters, and flameless combustors among others.
Numerous inventions for combustion heaters propose to heat formations by circulating hot gases or other fluids from surface heaters. For example, U.S. Pat. No. 6,056,057 proposes to produce a stream of hot gases in a surface burner and to then circulate those gases into the ground through a set of annular tubes cemented into bore holes. U.S. Pat. No. 3,237,689 proposed using a nuclear reactor to heat an oil shale formation. Aside from the obvious difficulties and doubtful economics of using nuclear power to produce oil, this process also required that heat be imparted to the ground through the circulation of a hot working fluid through a heat exchanger. Molten sodium circulating through a coiled pipe was proposed as the preferred embodiment.
U.S. Pat. No. 4,694,907 proposed circulating steam from a turbo-driven electric generator. According to this invention, fuel was to be burned in a surface turbine that powered an electric generator. Then, steam was to be produced from the heat of the exhaust gases as they exited the turbine. This steam was then to be circulated into the ground to heat the desired formation. This invention has the considerable advantage of utilizing a heat source that might otherwise be wasted. This improves the overall thermodynamic efficiency of the heating process considerably.
While such heat circulating systems avoid the inefficiencies of electrical heaters, they nevertheless have considerable problems of their own. One such problem is that the working fluids are necessarily hotter at the top of the formation than at the bottom and therefore do not heat the formation uniformly. Such systems also require large mass flows and correspondingly incur large costs associated with pressurizing and pumping large volumes of fluids. The net result is that, other considerations aside, such systems are probably not feasible except for resources in relatively thin deposits. According to U.S. Pat. No. 6,056,057: xe2x80x9cInjection of heat using only combustion gases to depths of greater than about 200 to 400 feet may be relatively expensive.xe2x80x9d
Numerous other inventions in the art propose a variety of down-hole burners. In these embodiments, combustion gases and air are fed to burners that are positioned in the bore holes; combustion of the fuel takes place underground and the resultant heat warms the formation. A variety of problems have attended each embodiment of the prior art, and a succession of inventions have been made to address these shortcomings. It was found for example that burners with open flames, like those described in U.S. Pat. No. 2,902,270, produce areas of excess heat adjacent to the flame and areas of inadequate heat elsewhere. This results in uneven heating of the formation with poor results. Open flames, which can produce local temperatures of 1650 degrees C., also create uneven stresses and thermal erosion within the burners. These hot spots lead either to limited equipment life or to high expenses for exotic materials.
Various inventions have been proposed to elongate the burn zone to achieve uniform heating. For example, U.S. Pat. No. 3,113,623 proposes the use of an elongated gas-permeable tube to extend the length of the active combustion zone. One problem common to such elongated burners involves the mixing of fuel and air. If the air and fuel are mixed on the surface, there is a very real danger of auto-combustion when the temperature of the mixture rises above a critical point on its way to the burner. This problem limits the use of such elongated burners to thin formations under shallow overburden.
Other heaters have been devised to eliminate flames altogether. For example, U.S. Pat. No. 5,255,742 avoids mixing of fuel and air until they reach the point of desired combustion. Problems of carbon formation are addressed by the addition of steam and/or carbon dioxide to the fuel stream as coke suppressants. In a later embodiment of the invention, U.S. Pat. No. 5,862,858, the addition of a catalyst to the combustor is intended to remove the need for coke suppression. Such flameless combustors apply heat to the ground evenly and can be adapted to formations of considerable thickness and depth.
One of the main problems with combustion heaters of all types is that they must consume an outside source of fuel. This is true even of combustion heaters used to produce combustible vapors from hydrocarbonaceous deposits. Until the formation has been warmed sufficiently to establish production at the collection wells, there is no internal source of fuel, which must therefore be imported. From a practical standpoint this means that a producer must buy natural gas or some other fuel and burn it in the down-hole combustors until production is established. Purchase of this fuel amounts to a significant expense. In resources typical of the art, like oil shales, tar sands, and diatomite oil formations, this expense can continue for as long as two years. If we take, for example, a down-hole burner 500 feet long, producing heat at the rate of 700 BTU per foot per hour, and operating with a thermal efficiency of 90%, then that heater will require fuel at the rate of 390 cubic feet per hour, assuming natural gas at 1000 BTU ft3. At $4 per mcf for natural gas, this would entail a daily fuel cost of over $37. Operating the heater for two years would incur expenses in excess of $27,000. To bring a single square mile of oil shale into production might require 10,000 heaters. When the fuel cost per heater is multiplied by the large number of heaters required, it can be seen that this is a substantial front-end expensexe2x80x94$270 million per square milexe2x80x94and can represent a significant fraction of overall project costs.
Even after production of combustible gases has been established from a hydrocarbon formation, the cost of fuel for combustion heaters is still significant. Many hydrocarbon formations will yield substantial amounts of combustible gases when heated. Oil shale, for example, can, depending on how it is heated, yield 30 gallons of oil per ton of shale, plus a thousand cubic feet of high BTU gas. This gas can be sent back down the bore holes to fire the burners, but it cannot be taken as xe2x80x9cfreexe2x80x9d. Since the gas could be marketed for from $2 to $5 per million BTU, that value, minus transportation and other marketing charges, must be set as the xe2x80x9ccostxe2x80x9d of the gas.
The high cost of fueling combustion heaters is one of the reasons that this particular method of in-situ resource extraction has not yet achieved widespread adoption.
There is a need for a subterranean heater with greater efficiency in terms of net energy production and reduced energy cost for mineral extraction and other applications. The heater would preferably consume a gaseous fuel of the type generated by the subterranean formation being heated as a normal by-product of the operation being performed to avoid the need to import fuel.
Ideally, the heater would produce heat uniformly along its length, without risk of auto-combustion and would heat formation at a reduced net cost for fuel.
The present invention incorporates all of these advantages.
The present invention is a subterranean heater composed of fuel cells. In the preferred embodiment, the apparatus comprises a plurality of fuel cells assembled in a vertical stack, with conduits throughout the stack supplying the cells with fuel and air, and removing exhaust gases. Preferably, the fuel cell stack is enclosed in a casing adapted for insertion into a well bore. An electrical connection is provided to the far end (typically bottom) of the stack to allow completion of an electric circuit.
The encased fuel cell stack is inserted into a wellbore, preferably vertically, but potentially horizontally or at some other orientation. Preferably, the encased conduit is cemented into the borehole by a suitably heat conducting grout. Fuel and air are pumped into the stack through the incorporated conduits to the fuel cells. Within the fuel cells, electrochemical reactions take place to produce electricity and heat. The electricity passes out of the stack through an electric circuit. Fuel cells, of the solid oxide type, which are preferred, operate at temperatures in the 800 to 1000 degree Centigrade range. This is also the preferred temperature range for many subterranean heating applications. Heat passes from the fuel cell stack to the underground formation by thermal conduction. Thus, the operating fuel cell stack acts as a down-hole conduction heater.
In the preferred embodiment of the invention, conduits for air, gaseous fuel, and exhaust are formed by aligning holes in plates supporting the fuel cells. Communication for circulation of these gases is provided by channels formed in the surface of the plates connecting the conduits to the fuel cells.
The size and activity of the fuel cells themselves can be modified to tailor the output of thermal energy to the formation being developed. Alternatively, the fuel cells themselves may be standardized, to maximize production efficiency, while the thermal properties of the stack are varied by the insertion of spacers between active fuel cells.
A refinement of the invention may be to include a manifold connecting the fuel cell stack to the surface. This manifold may be insulated to minimize heating of overburden above the resource deposit. Further, the manifold may serve as a heat exchanger between the exhaust gases leaving the fuel cell stack and the incoming streams of fuel and/or air. By this method, the maximum amount of thermal energy is retained within the target formation.
Where the invention is used to produce hydrocarbonaceous resources, it is intended that the volatile gases, produced as the result of heating such deposits, should be used as fuel to power the fuel cells. By this method, the fuel cells will be self sustaining. Since the thermal process produces a fuel stream, and the fuel is first converted to electricity in the fuel cells, the production cycle is therefore being powered by that fraction of total energy that is otherwise usually wasted. The net result is an increase in the overall thermodynamic efficiency of the resource extraction system.
The fuel cell heater does double duty as both a heating element and a power generator, resulting in increased economic efficiency. The present invention overcomes many of the diseconomies of other subterranean heaters by reducing the cost of heat produced by the fuel cells to essentially zero. Although the fuel cells do require fuel, the cost of fuel is offset by the value of the electricity the cells produce.
The present invention combines the advantages of down-hole combustion heaters with the advantages of electrical resistance heaters, while eliminating most of the disadvantages typical of subterranean heaters of the prior art. The present invention converts fuel to heat, like combustion heaters, avoiding the inefficiencies of electrical resistance heaters. The present invention produces heat uniformly over the length of the heater, like electrical resistance heaters, while avoiding the hot spots and uneven heating of combustion heaters. The present invention also eliminates the problems associated with mixing fuel and air in flameless combustor heaters by maintaining separation of these gases across the fuel cell electrolyte.